Acrocerid flies (Diptera, Brachycera) are endoparasitoids of
spiders. Each larval instar is morphologically unique and has a
distinctive lifestyle (hypermetamorphosis: Schlinger 1987). Their
planidial first instar larvae actively seek their spider host or, only
in the genus Acrocera, attach themselves to the substrate where they
have hatched, waiting for a host spider to pass by (Schlinger 1987,
2003; Nielsen et al. 1999). Once a host is found, the planidium climbs
on to the spider, migrates to the spider's abdomen, and cuts a
small hole to enter the spider en route to the booklungs (Schlinger
1987; see Nielsen et al. 1999, for an alternative strategy to enter the
host). In the booklungs, the larva molts again, attaches itself to a
booklung, and enters a resting stage. After molting, the fourth instar
larva feeds actively inside the spider and causes the parasitized spider
to spin a molting-web like retreat. The acrocerid larva then emerges
from the spider, finishes feeding, fixes itself to the web and pupates
(Schlinger 1987). Acrocerid flies show a preference for wandering,
fossorial, and web-building spiders that live close to the ground and
wander in adjacent vegetation (Cady et al. 1993).

We report new spider (Araneae) host associations for Ogcodes
melampus Loew 1872, O. eugonatus Loew 1872, and Acrocera sp. (Group IV;
sensu Sabrosky 1944) (Diptera: Acroceridae). Foliage spiders were
sampled by beating live and dead branches between 10 May and 24
September 2007 in the canopy of mature trees and understorey saplings of
sugar maple (Acer saccharum Marsh.) and American beech (Fagus
grandifolia Eher.) at the Morgan Arboretum, Sainte-Anne-de-Bellevue,
Quebec, Canada (45[degrees]25'55"N;
73[degrees]56'58"W). In the laboratory, the spiders were
housed individually in small plastic containers and kept alive in
preparation for a ballooning dispersal experiment.

During this experiment, cream yellow pupae were noticed inside
containers of three, dead, sub-adult individuals of Pelegrina proterva
(Walckenaer 1837) (Araneae: Salticidae, body size = 3.9 mm, n = 4). Two
of these individuals of P. proterva were sampled in the canopy of mature
American beech trees and one in the canopy of a mature sugar maple on 7
June 2007. Adult flies emerged in the plastic containers approximately
28 days later, in early July 2007. The adults were determined to be two
females of O. eugonatus (one from American beech and the other from
sugar maple) and one female of O. melampus (from American beech).

In similar fashion, a female Acrocera sp. (Group IV, near female 1
sensu Sabrosky 1948) emerged from a sub adult individual of Eris
militaris (Hentz 1845) (Araneae: Salticidae, body size = 5.2 mm, n = 6).
This individual of E. militaris was sampled on an American beech sapling
in the understorey on 3 July 2007, the acrocerid larva had pupated a
week later, and the adult fly emerged approximately 2 wk later. Overall,
0.88 percent of the spiders in our study were parasitized by
Acroceridae.

Acrocera is known to lay its eggs on grass stems (Schlinger 1987),
potentially not far removed from American beech saplings. Eris
militaris, the host spider, is also significantly associated with the
understorey layer in this habitat type (Larrivee & Buddle 2008). In
contrast, the three infected individuals of P. proterva originated from
the canopy. Females from the genus Ogcodes lay their eggs on the tips of
dead twigs (Schlinger 1987), common in the canopy of beech trees. Only
four acrocerid parasites were found in our study but the rarity of these
flies makes this an important life history observation. Ogcodes
specimens were only in spiders from the canopy and the Acrocera specimen
in an understory spider. Future research on hardwood forest Ogcodes and
Acrocera species should test their potential preference for canopy and
understorey spiders respectively.

Ogcodes melampus is mainly found in the western part of North
America with previous records placing it as far east as Minnesota
(Schlinger 1960). This specimen represents a significant range extension
for this species. Other northeastern specimens of O. melampus were found
in the Canadian National Collection in Ottawa, from both Michigan and
Ontario. There is no life history available for this species (Schlinger
1960) and it has been reared from only two spider species, a lycosid and
a thomisid (Schlinger 1987). Our record adds the family Salticidae and
the species P. proterva to its host list. Ogcodes eugonatus has been
reared from Lycosidae, Oxyopidae, Thomisidae, and Salticidae (species
are listed in Schlinger 1987) though this is the first record of this
species from a P. proterva host.

The genus Acrocera occurs across North America though records from
Acrocera Group IV (sensu Sabrosky 1944) mostly originate from eastern
North America. They are known endoparasitoids of seven spider families:
Plectreuridae, Lycosidae, Agelenidae, Amaurobiidae, Clubionidae,
Gnaphosidae, and Salticidae (Schlinger 1987). Acrocera bulla Westwood, a
member of Group IV, is the only other known species from the genus
Acrocera that is an endoparasitoid of the family Salticidae. Our
observation adds E. militaris to the host list of spiders for the genus
Acrocera.

We thank Chris Buddle for his support of this project including the
use of the DINO 260xt mobile aerial platform to access the tree canopies
(Canadian Foundation for Innovation New Opportunities Grant (Project
#9548). Cristina Idziak allowed us to sample in the Morgan Arboretum.
Jeff Cumming from the Diptera section at the Canadian National
Collection kindly provided determined specimens of Ogcodes and Acrocera
for comparison. Finally, we thank Robb Bennett and two anonymous
reviewers for comments on an early draft of the manuscript.